1. Field of the Invention
The present invention relates to an apparatus for measuring the reflectivities of the resonator facets of a semiconductor laser when the facets are covered with a protective coating of dielectric material or the like.
2. Description of the Prior Art
Semiconductor lasers having varying light reflectivities have been developed wherein the facets of the resonator are covered with a protective coating of dielectric material or the like in order to prevent deterioration of the facets and to control the light reflectivities of the facets within predetermined values. Conventionally, the reflectivities of the resonator facets thus coated for protection are determined by measuring the optical output power from the front facet and the optical output power from the back facet and calculating the reflectivities from the following equation which represents the relation between these output powers and the reflectivities. ##EQU1## wherein P1: light output power from one facet
R1: light reflectivity of one facet PA1 P2: light output power from the other facet PA1 R2: light reflectivity of the other facet
(As to Equation (1), see IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. QE-19, No. 3, March 1983. )
With this method of calculation, however, it is impossible to determine the reflectivity of one facet, i.e., one of R1 and R2, unless the reflectivity of the other facet is known. Accordingly, it is common practice to make one of the resonator facets assume the state of a cleaved plane having a known reflectivity, to form a protective coating only over the other facet to provide an altered reflectivity and to calculate the reflectivity of the coated facet from Equation (1). In the case where both facets are provided with a protective coating, a monitoring semiconductor laser is used in which one facet is in the form of a cleaved plane as described above and the other facet is provided with a protective coating under the same condition as the coating concerned, and the reflectivity of the coated facet is determined from Equation (1). It is therefore impossible to directly measure the facet reflectivities of individual semiconductor lasers and to check the variations involved in the manufacturing process.